The construction of a pile foundation requires a careful choice of piling system depending upon the subsoil condition and load characteristics of the structure. The permissible limits of total and differentials settlement, unsupported length of the pile under scour and any other special requirements of the project are also equally important criteria for adoption.
For piles in streams, rivers, creeks, etc the following criteria may be followed:
- Scour conditions are properly established
- Permanent steel liner should be provided at least up to maximum scour level. In the case of marine clay or soft soil or soil having aggressive materials, a permanent steel liner of sufficient strength shall be used for the full depth of such strata. The minimum thickness of the liner should be 6 mm
- For bridge located inland, steel liners of a minimum thickness of 6 mm shall be provided in the case given below. The liner shall be provided up to depth up to which following situations prevail:
- While constructing the pile foundation through very soft clay (N<3), very loose sandy strata (N< 8), boundary formation and artesian condition, wherein the walls of boreholes cannot be stabilized by bentonite
- Where sewage leakage is a common phenomenon as well as sites with aggressive soil/water environment.
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Guideline to Structural Design of Piles
A pile structural member shall have sufficient strength to transmit the load from structure to soil. The pile shall also be designed to withstand temporary stresses if to which it may be subjected to such as handling and driving stresses. The permissible stresses should be as per IRC 112 (0.45 fck)
The test pile shall be separately designed to carry the test load safely to the foundation.
The piles may be designed taking into consideration all the load effects and their structural capacity examined as a column. The self-load of a pile or lateral load due to earthquake, water current force, etc. on the portion of free pile up to scour level and up to potential liquefaction level, if applicable, should be duly accounted for.
Reinforcement for cast-in-situ piles
Area of longitudinal reinforcement shall not be less than 0.4 per cent nor greater than 2.5 per cent of the actual area of the cross-section in all cast in situ concrete pile. The clear spacing between the vertical bars shall not be less than 100 mm. Grouping of not more than two bars together can be made for achieving the same. Lateral reinforcement shall be provided in the form of a spiral with a minimum 8 mm diameter steel, spacing not more than 150 mm. For an inner layer of reinforcement, separate links tying them to each other and to outer layers shall be provided.
Design of Pile Cap
A minimum offset of 150 mm shall be provided beyond the outer face of the outermost piles in the group. If the pile cap in contact with the earth at the bottom, a levelling course of a minimum of 80 mm thick plain cement concrete shall be provided.
The top of the pile shall project 50 mm into the pile cap and reinforcements of the pile shall be fully anchored in the pile cap.
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In Marine condition or in areas exposed to the action of harmful chemicals, the pile cap shall be protected with suitable anti-corrosive paint. The high alumina cement i.e. quick-setting cement shall not be used in marine constructions.
The minimum thickness of the pile cap should be 1.5 times the diameter of the pile. Such a pile cap can be considered rigid. The pile cap may be designed as a thick slab or by using the “strut and tie” method. All reinforcement in pile cap shall have full anchorage capacity beyond the point at which it is no longer required. It should be especially ascertained for pile cap designed by the “strut and tie” method. Where the larger diameter of bars are used as main reinforcement the corners of the pile cap have large local cover due to the large radius of bending of main bars. Such corners shall be protected by locally placing small diameter bars.